Deposition and Provenance of Miocene Intraclastic Chalks, Blake-Bahama Basin, Western North Atlantic

Abstract

The Miocene Great Abaco Member of the Blake Ridge Formation, cored at Sites 391 and 534 in the Blake-Bahama Basin, consists predominantly of redeposited deeper-water foraminifer-nannofossil chalk and minor shallow-water carbonate material, interbedded with radiolarian-diatom mudstone deposited below the calcite compensation depth. The mudstones comprise the background rain of pelagic sediments that may have been deposited and/or reworked by weak contour currents. The chalks, including the distinctive intraclastic chalks, and skeletal packstones were deposited by turbidity currents, and less so by debris flows. These gravity flows eroded and entrained large numbers of siliceous mud and clay intraclasts, as well as lithified chalk and limestone clasts, some of shallow-water origin. Possible sources of the carbonates include the Blake Plateau to the west and the Bahama Banks to the south. The Blake Plateau and Bahamian slopes provided the deeper-water foraminifer-nannofossil chalks, and the Bahama platform supplied the shallow-water carbonate grains and cemented limestone. Sea-level changes together with tectonism along the Great Abaco Fracture Zone and faults in the northern Bahamas may have provided the triggering mechanism that set large volumes of mostly unconsolidated carbonate material into motion. A submarine fan system and associated channels extending from the northern Bahamas into the Blake-Bahama Basin funneled debris flows and turbidites over 300 km into the basin, and subsequent turbidites filled in and leveled the topography, creating the prominant seismic reflector M. If similar deposits existed earlier in the Paleogene, they have been eroded, leaving only the Great Abaco Member as evidence of the tectonic and sedimentologic conditions existing in the western North Atlantic during the Paleogene and Miocene.

Site 628: Little Bahama Bank

“…The thickness and grain size of sediment gravity flows in crease upward in Unit I, the marginal rise being at the foot of the platform over the pelagic deposits of the Blake Plateau. The slump and associated turbidites at the base of Unit I are coeval with the gravity flows of the Abaco Member (Bliefnick et al, 1983) and may be related to a regional (probably tectonic) event.…”

Seismic Stratigraphic Implications of ODP Leg 101 Site Surveys

“…Sequence Boundary 9/8: The only well-sampled part of the section penetrated at Site 626 is a series of middle Miocene debris flows and turbidites (see Austin, Schlager, et al, 1986) coeval with the Great Abaco Member of the Blake Ridge Formation sampled during DSDP Legs 44 and 76 in the Blake-Bahama Basin to the north Bliefnick et al, 1983). These Miocene deposits correlate with seismic sequence 9 (Austin, Schlager, et al, 1986).…”

Leg 101—An Overview

“…Furthermore, Harwood and Towers (this volume) demonstrate that the large slumps on the Little Bahama Bank slope are of middle Miocene age. All these sedi ments are coeval with intraclastic chalks of the Abaco Member of the Blake Ridge Formation just to the north in the Blake-Ba hama Basin (Benson, Sheridan, et al, 1978;Bliefnick et al, 1983). They may also have been deposited at the same time as the collapse of part of the West Florida Escarpment (Mullins et al, 1986).…”

“…(Watkins et al, this volume). Second, the recov ery of a sequence of middle Miocene debris flows coeval with the intraclastic chalks of the Great Abaco Member of the Blake Ridge Formation sampled in the Blake-Bahama Basin to the north during DSDP Legs 44 (Benson, Sheridan, et al, 1978) and 76 Bliefnick et al, 1983) suggests a widespread triggering mechanism (Fulthorpe and Melillo, this volume), hereafter termed the "Abaco event." Third, Site 626 documents that the seismic stratigraphy in the straits calibrated by Sheridan et al (1981) using jump correlation to the Great Isaac 1 borehole approximately 60 km to the north east ( Fig.…”